To a significant extent it's the unequal exchange rates.
Countries like the US, the EU, Canada, Australia, all have advnaced economies which can generate wealth internally. The US inparticular, is a power house in this respect. The UK's economic growth is almost entirely based upon financial services, which generate enormous wealth.
Most countries do not have very good internal wealth generation. The reason is quite simply, corruption. Though there are other factors, this is paramount. If you look at any of the more reliable corruption indicies, there is a direct corelation between levels of internal corruption and wealth generation.
So, these countries need to earn their incomes almost entirely from exports. To ensure their products sell, they have to charge lower prices. That means devaluing their currencies.
An indication of this can be found by looking at the man hours worked in relation to the price of goods. Surprisingly, in most countries, this averages out, even in wealthy countries such as the US and the EU.
But when we go there with our dollars and Euros, we get very favourable exchange rates, so it appears that we are wealther than we actually are.
